Caliban, or Uranus XVI

Caliban was discovered with the 5m Hale telescope at Palomar in September 1997. It is a rather small body, estimated at only 72km in diameter, and is a dark reddish colour with an estimated albedo (i.e. reflectivity) of only 0.04 — about half that of the Moon. No high resolution images exist because it was not discovered until long after the Voyager 2 fly-by mission.

Because Caliban is so small, so dark and so far away it appears very faint as seen from the Earth. At the time when the image was taken the predicted magnitude was 22.2 so a long exposure time was required to make the satellite visible at all. Luckily Uranus and its moons were near a stationary point in its apparent orbit and Caliban moved less than two arc seconds during the three hour observation, making stacking the numerous sub-images very easy. Even with an exposure that long Caliban is at the limit of detectability with the equipment at Tacande. The signal to noise ratio was barely 3 and careful contrast enhancement was required before the satellite became clearly visible. Even so, it is not blindingly obvious! Incidentally, the linear streak across the bottom left of the image was caused by a passing artificial satellite intruding on one of the sub-images.

Some people find it easier to see very low contrast star-like objects if they appear dark on a light background, others the reverse. Both are presented above so you can decide for yourself.

The object in the image is known to be Caliban, as opposed to a background star or an asteroid, for three reasons. First, it lies within 2 arcsec of the position predicted by the Minor Planet Center ephemeris. Second, there were no asteroids known to the MPC anywhere near that position at the time of observation. Third, images taken as part of the second Palomar Observatory Sky Survey (POSS2) show nothing at that location.